Hymatomelanic acids

Hymatomelanic acid Alcohol-soluble portion of humic acid... 

The numbers of humic fractions continued to increase, and terms such as mucic acid, lignoic acid, and hymatomelanic acid were introduced all of these were considered to represent chemically individual compounds, which of course they were not. Sprengel (1826) promoted fractionation on the basis of solubility characteristics... 

It would be pointless to draw up a classification system that takes account of several fractions based on charge density differences, or even differences in solubilities in organic solvent systems. Consideration might be given to the hymatomelanic acid, or the alcohol-soluble component described by Hoppe-Seyler (1889). It would be important to distinguish between the FA fraction (or the material that is soluble in acidic and basic media) and the FAs as defined by the IHSS (or the fractions recov-... 

Specific organic matter or humic substances These are produced by the connection of decompositions products. Their structure is not exactly known, and depends on production conditions. On the basis of solubility, humic substances are classified as fulvic acid, humic acid, humin, and hymatomelanic acid (Stevenson 1982). 

The above scheme provides further fractionation of humic acids allowing the separation of a fraction known as hymatomelanic acid and of fulvic acids in several fractions (B, D, etc.). Humin is commonly further purified of inorganic components. The elemental composition of the fractions obtained with this separation scheme from an uncultivated prairie brown soil (TypicXerochrept) is given in Table 14.1.1. 

Hymatomelanic acid Soluble in alkali, precipitated by acid, soluble in alcohol Hymatomelanic acid Hymatomelanic acid ... 

With regard to the designation of humin as a separate fraction, it is possible that this material consists of portions of other fractions so intimately associated with mineral matter that they cannot be solubilized by extraction with alkali. Also, it is not known whether hymatomelanic acid is a distinct chemical entity. This material may be an artifact produced from humic acid during fractionation. The simple process of redissolving the alcohol-insoluble material in alkali followed by reprecipitation with acid results in a further increase in alcohol-soluble material. 

Organic solvents have long been used for extraction and sequential extraction, which is fractionation of a sort (Flaig et al., 1975 Schnitzer, 1978). While the direct use of organic solvents in fractionation has not been widespread, nonetheless, the technique has received some attention. For instance, the separation of hymatomelanic acid from precipitated humic acid is obtained by extraction with ethanol (Oden, 1919). Ethanol has been used to bring about fractional precipitation by addition to alkaline solutions of humic acid (Kyuma, 1964 Kumada and Kawamura, 1968). There is no reason why other water-miscible solvents such as acetone and methanol should not be used in this way. Solvents that are highly immiscible with water (e.g., hexane and benzene) do not appear to remove any substantial fraction of humic substances. These are perhaps best used to remove nonhumic substances (such as fats and waxes) prior to extraction. However, recent work by Allen and MacCarthy (personal communication) has shown that more polar water-immiscible solvents, such as methyl isobutyl ketone and diethyl ether, can be used successfully to purify and fractionate humic substances. 

Fulvic acids are organic substances which remain in the solution after acid precipitation of humic and hymatomelanic acids from the extract of humic acids. The chemical structure and physico-chemical properties of these substances are similar to those in humic acids. The aromatic character of the molecule is less distinct, and side chains predominate. They also differ from humic acids in their lower molecular weight, yellow to light-brown colour, better solubility in water and easier hydrolysis in acid media. They are easily eluted from soils and peat. They are more acid than humic acids (pH of solution with c = 0.01 mol 1 is about 2.6). They manifest complex-forming properties and exchange capacity. 

Table 3.19. Comparison of functional groups in hymatomelanic acids and humic acids...

Fundamental structure Hymatomelanic acids Humic acid... 

During the first half of the twentieth century renewed efforts were deployed to classify organic substances and to determine their chemical nature and structure. Most of the approaches proposed classification based on solubility characteristics humic acid (soluble in alkali, insoluble in acid), fulvic acid (soluble in alkali and acid), hymatomelanic acid (alcohol-soluble part of humic acid), humin (insoluble in alkali). 

Other groups include, for example, hymatomelanic acid, the alcohol-soluble fraction (Senesi and Loffredo 1998). Clearly, these fractions are loosely defined and can include many different compounds. Usually, HSs are obtained from the soil through alkaline extraction, which separates FA and HA but not humin, albeit other procedures have also been proposed (Swift 1996). The structure and properties of HSs are considered in Chapter 10. 

Johnson, the carbo-ulmic acid of Herz and the hymatomelanic acid of 54... 

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